The present invention relates to preparations with predetermined partial pressure of O.sub.2 and CO.sub.2, particularly useful for calibrating analytical instruments commonly used for measuring blood gases.
By instruments for measuring blood gases there are herein meant apparatus for diagnostic purposes adapted to measure the partial pressure of O.sub.2 and CO.sub.2 in venous or arterial blood, or mixtures thereof, and the pH of the same. Such instruments may also measure other parameters, and especially ions such as sodium, potassium, calcium and chloride and other analytes in whole blood such as glucose.
Instruments of this kind are normally used in hospital laboratories, intensive care units, etc., and are in particular surgical conditions indispensable, in that no other instrumental devices can fulfill the functions that they are capable of fulfilling.
On the basis of the three fundamental parameters measured, derived variables can be determined. The most significant and important derived parameter is the acid/alkali balance (an indicator of lung and renal function). Since the 1950's, many aspects of the said apparatus have evolved technologically in the interests of greater simplicity of use, automation and dependability.
Nevertheless, apparatus for measuring blood gases remain complex and delicate and, if they are to operate correctly, require frequent calibration with secondary standards. Currently provision is made for buffers of known pH for calibrating the pH channel, and mixtures of gases of known percentage for calibrating the pO.sub.2 and pCO.sub.2 channels. These gas mixtures (O.sub.2, CO.sub.2, remainder nitrogen) are generally contained in pressurized cylinders.
Certification of gas cylinder content is very costly and calls for equipment not available everywhere. Gas-mixers requiring the use of 100% CO.sub.2 and ambient air have been designed and constructed to obviate this difficulty; but the problems of transportation, bulk and availability of the said cylinders remains.
Fluorocarbon emulsions, such as are described in U.S. Pat. Nos. 4,151,108, 4,163,734, 4,116,336 and 4,722,904 have been used as controls and are proposed in these patents for use as calibrants. For a variety of technical reasons, involving especially calibration of the pH electrode, fluorocarbon emulsions are not used commercially as calibrants.
The object of the present invention is to provide a solution for this problem of a kind that also simplifies the apparatus by enabling the calibrant to be maintained in the liquid phase. The apparatus can then be calibrated by means of liquid phases with pre-determined and stable partial O.sub.2 and CO.sub.2 pressures.